This invention relates to semiconductor processing equipment, and more particularly to apparatus for performing high-temperature processes upon semiconductor wafers in a tube furnace.
Various types of thermal processing equipment are available commercially. During a process cycle, several boats containing silicon or other kind of wafers, supported by some kind of rod, are inserted into the furnace where the desired process is performed. Upon completion of the process cycle, the wafers are removed from the furnace and unloaded from the supporting rod. During the insertion and removal steps of the process, it is preferred that the wafers be protected from exposure to ambient air. Oxygen, water vapor and airborne particulates found in ambient air can react with the wafers and adversely affect their chemical and physical properties. One very convenient apparatus to do this has been described in U.S. Pat. Nos. 4,459,104 to Wollman and 4,543,059 to Whang et al, which describe a tubular cantilever into which the loaded wafer boats are inserted. By means of the tube, the wafers are protected against particulates, and, by flowing an inert gas through the tube, also protected from moisture and air. This is particularly important during the cantilever insertion and removal steps of a typical thermal process cycle. The tubular cantilever also reduces the amount of contaminants seen by the wafers inside the furnace tube itself since the wafer is isolated by the tubular cantilever from the furnace tube.
Further, it is important that the reaction or other processing step being performed on the wafers be carried out uniformly on all the wafers being processed, and uniformly across the surface of each wafer. Various furnace design changes have been made to assure a uniform flow of the reaction gases through the furnace tube to prevent contaminants from entering the reaction tube and adversely affecting the wafers, and to ensure efficient removal of the gases fed to the furnace.
Previously, the problems caused by ambient air infiltration in furnaces of this type have been addressed either by using high purging gas rates through the furnace tube from the nozzle at the opposite end, to create a positive pressure inside the furnace, or by using high scavenger exhaust rates to create a partial vacuum inside the scavenger area and thus to reduce air infiltration by diverting it into the scavenger exhaust. The scavengers are usually asymmetrical (i.e., located on one side of the furnace tube), so with high flow rates of purge gas through the reactor nozzle the exiting gas flow is also asymmetrical and can allow ambient air to enter due to the pressure gradients introduced by these asymmetries.